A key green energy initiative is the discovery of efficient, stable, and elemental abundant electrocatalysts for the water splitting reactions, i.e. the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).1-3 Water splitting with HER and OER electrocatalysts play a vital role in converting solar energy into chemical energy via artificial photosynthesis, and also provides a pathway to use water (as opposed to natural gas) as a feedstock for hydrogen production.4-6 Nanostructured transition-metal chalcogenides have previously been studied as HER electrocatalysts.7-15 While other transition-metal chalcogenides have also been studied as HER catalysts (e.g. WS2, NiS2, COS2, NiSe2, and CoSe2),8,16 there have been only limited reports on the catalytic activity of FeS2,8,17 and none have shown high efficiency for FeS2.